In any device containing a helical spring which is flexed to store energy, the helical spring will "wind-up" as the spring is compressed, and unwind as the spring is allowed to expand. This winding of the spring imparts torque to the parts constraining the ends of the spring, and to any members attached to the spring constraining parts. An example of a device exhibiting this wind-up condition is an upper spring assembly of an electromagnetic actuator.
A conventional electromagnetic actuator for opening and closing a valve of an internal combustion engine generally includes "open" and "close" electromagnets which, when energized, produce an electromagnetic force on an armature. The armature is biased by a pair of identical upper and lower springs arranged in parallel. The armature is coupled with a gas exchange valve of the engine. The armature rests approximately half-way between the open and close electromagnets when the springs are in equilibrium. When the armature is held by a magnetic force in either the closed or opened position (at rest against the open or close electromagnet), potential energy is stored by the springs. If the magnetic force is shut off with the armature in the opened position, the spring's potential energy will be converted to kinetic energy of the moving mass and cause the armature to move towards the close electromagnet. If friction is sufficiently low, the armature can then be caught in the closed position by applying current to the close electromagnet.
Generally, the upper helical spring of the actuator is in a compressed state and is allowed to expand during operation of the actuator. The upper helical spring is generally constrained at one end by a spring adjusting screw and by a shaft assembly at the other end of the spring. The shaft assembly is coupled to the armature of the actuator. Thus, the upper helical spring can undesirably cause torque to be transmitted to the armature, which should not rotate. One method of preventing rotation of the armature is to provide some guiding mechanism for the armature. However, in a device such as an actuator which operates millions of strokes in its lifetime, a guiding mechanism can wear out quickly.
Accordingly, a need exists to provide a low cost spring assembly which allows one end of the spring to rotate during expansion thereof, yet substantially prevents torque from being transmitted to a member engaged with the other end of the spring.